Inkjet printing processes fall into two main types: continuous processes and drop-on-demand (DOD) processes. Continuous processes use electrically conductive ink to produce a stream of drops of electrically charged ink that are deflected by an electric field to an appropriate location on a substrate. In DOD processes, individual drops of ink are expelled from the nozzle of a print head either by vibration of a piezoelectric actuator (in piezoelectric inkjet printing) or by heating the ink to form a bubble (in thermal inkjet printing, also known as bubble jet printing). Thermal inkjet printing has advantages over piezoelectric inkjet printing, with printers and print heads being lower cost and with the printing process being able to achieve better resolution.
Inkjet inks need to satisfy a number of requirements, including the following:                Viscosity must be appropriate. With DOD inks there are greater limitations on inks for thermal printing than for piezoelectric printing, with it generally being necessary for in to have a viscosity of below about 4 mPa·s at print head operating temperature (which is typically 40-50° C.), which usually equates to a viscosity of less than 6.5 mPa·s at room temperature (25° C.), to be capable of being thermally inlet printed. In this specification, all viscosity values are at 25° C. unless otherwise specified.        The ink must not cause unacceptable levels of clogging or blockage of printing nozzles.        The ink must not result in build up of deposits on the ejection heaters of thermal inkjets print heads (a process known as “kogation”) to an unacceptable level during the working life of a print head.        The ink should be stable in storage, without settling out or coagulation of materials.        The resulting print needs to saws desired characteristics depending on the field of use, with possible relevant factors including water fastness, scratch resistance, durability, lack of shrinkage, lack of cracking, flexibility, optical density (for coloured inks), uniformity of deposition.        
Curable DOD is are known. These typically comprise one or more monomers etc. curable in response to appropriate conditions, typically ultra violet (UV), infra red (IR) or heat.
U.S. Pat. No. 5,623,001 (Scitex) discloses TV-curable DOD inkjet inks, particularly for piezoelectric printing, comprising water (20-75%) and water-miscible UV-curable monomer and/or oligomer e.g. acrylic materials (20-60%). The document makes no reference to thermal inkjet printing, and does not teach how to produce inks with a viscosity of less than 6.5 mPa·s, suitable for thermal inkjet printing, nor does it teach how to prevent kogation.
U.S. Pat. No. 5,952,401 (Canon) discloses curable water-based inks for piezoelectric and bubble jet printing using curable monomers. The document does not address the issue of prevention of kogation caused by curable materials.
U.S. Pat. No. 6,294,592 (BASF) discloses curable inkjet inks exemplified by UV-curable polyurethane dispersions. The document does not address issues of viscosity or prevention of kogation.
Curable monomers and oligomers tend to have limited solubility/miscibility in water, and substantial practical difficulties arise in producing water-based inks with sufficiently low viscosity (below 6.5 mPa·s) to be useful for thermal inkjet printing that do sot undergo undesirable phase separation, do not cause clogging or blockage of printing nozzles, do not result in unacceptable levels of kogation, and that produce prints with appropriate, useful properties.
We have found that by use of carefully selected mixtures of curable materials together with a water-compatible solvent (referred to as a co-solvent) for the curable materials it is possible to produce low viscosity compositions suitable for inkjet printing. We have also found various materials that are effective in preventing or reducing kogation, referred to herein as anti-kogation agents.